Provided is a 4-transistor CMOS image in which a driving condition or a pixel structure is changed so that a transfer transistor in a pixel operates in a pinch-off condition during reset and transfer operations in order to reduce dark current and fixed-pattern noise caused by a change in an operation condition of the transfer transistor and inter-pixel characteristic discrepancy. The image sensor includes a photosensitive pixel including a transfer transistor for transferring photon-induced charges created in a photodiode; and a voltage control unit for controlling a turn-on voltage applied to a gate of the transfer transistor to be lower than a floating diffusion node voltage plus the threshold voltage of the transfer transistor during a partial or entire section of a turn-on section of the transfer transistor such that the transfer transistor operates in a pseudo pinch-off mode.
Legal claims defining the scope of protection, as filed with the USPTO.
1. An image sensor comprising: a photodiode comprising an n-doped region and a surface p-doped region; a floating diffusion node composed of an n+-doped region; and a transfer transistor including: a gate electrode a gate oxide layer beneath the gate; a channel underneath the gate oxide layer such that the gate oxide layer is interposed between the gate electrode and the channel, the channel comprising a portion of a p-substrate; a p-type doping portion in which holes are accumulated formed contiguous to and beneath the gate oxide layer of the transfer transistor and adjacent to the surface p-doped region of the photodiode so as to be between the surface p-type region and the channel, wherein the p-doped region of the photodiode is between the gate oxide layer and the n-doped region of the photodiode, and wherein the transfer transistor for transferring photon-induced charges created by the photodiode to the floating diffusion node, wherein the floating diffusion node and a channel of the transfer transistor are separated by a depletion region when the transfer transistor is turned on, so that the transfer transistor operates in a pseudo pinch-off mode.
2. The image sensor of claim 1 , wherein a switching signal having a negative (-) offset voltage applied to the gate electrode during a partial or entire section of a turn-off section of the transfer transistor causes the holes to be accumulated.
3. The image sensor of claim 2 , wherein the negative offset voltage ranges from -0.1V to -1.0V.
4. The image sensor of claim 1 , wherein the p-type doping portion has a different doping pattern than the surface p-type region of the photodiode.
5. The image sensor of claim 1 , wherein the floating diffusion node and the channel of the transfer transistor are separated by adjusting a process condition or layout during a manufacturing process of the image sensor.
6. The image sensor of claim 5 , further comprising a reset transistor for resetting the floating diffusion node, wherein the threshold voltage of the transfer transistor or the reset transistor is changed through a process condition modification so that the transfer transistor operates in a pseudo pinch-off mode.
7. The image sensor of claim 5 , further comprising a reset transistor for resetting the floating diffusion node, wherein the threshold voltage of the transfer transistor is made higher than the threshold voltage of the reset transistor by ion implantation or oxide layer thickness adjustment.
8. An image sensor comprising: a photodiode; a transfer transistor for transferring photon-induced charges created by the photodiode to a floating diffusion node; a reset transistor for resetting the floating diffusion node; a pulse generating block applying a first turn-on voltage to a gate of the transfer transistor and a second turn-on voltage to a gate of the reset transistor, wherein the pulse generating block is configured to apply the first and second turn-on voltages such that the floating diffusion node and a channel of the transfer transistor are separated by a depletion region when the transfer transistor is turned on during a period in which photon-induced charges accumulated in the photodiode are transferred to the diffusion node so that the transfer transistor operates in a pseudo pinch-off mode.
9. The image sensor of claim 8 , wherein the threshold voltage of the transfer transistor has a first threshold voltage and the reset transistor has a second threshold voltage, and one of the first and second threshold voltages is increased or decreased with respect to the other of the first and second threshold voltages; and wherein the pulse generating block applies first and second turn-on voltages having the same level such that the floating diffusion node and the channel of the transfer transistor are separated by the depletion region when the transfer transistor is turned on during the period in which photon-induced charges accumulated in the photodiode are transferred to the diffusion node.
10. The image sensor of claim 9 , wherein the threshold voltage of the transfer transistor is made higher than the threshold voltage of the reset transistor.
11. An image sensor comprising: a photodiode; a transfer transistor for transferring photon-induced charges created by the photodiode to a floating diffusion node, the transfer transistor including a gate and a gate oxide layer and having a structure capable of causing holes to be accumulated beneath the gate oxide layer, a reset transistor for resetting the floating diffusion node; wherein the floating diffusion node and a channel of the transfer transistor are separated by a depletion region when the transfer transistor is turned on, so that the transfer transistor operates in a pseudo pinch-off mode a pulse generating block applying a constant negative offset voltage to the gate of the transfer transistor such that the holes are accumulated beneath the gate oxide layer during a first period, and applying first and second turn-on voltages to the transfer transistor and the reset transistor respectively, such that the floating diffusion node and a channel of the transfer transistor are separated by a depletion region when the transfer transistor is turned on so that the transfer transistor operates in a pseudo pinch-off mode during a second period.
12. The image sensor of claim 11 , wherein the negative offset voltage ranges from -0.1V to -1.0V.
13. The image sensor of claim 11 , wherein the threshold voltage of the transfer transistor has a first threshold voltage and the reset transistor has a second threshold voltage, and one of the first and second threshold voltages is increased or decreased with respect to the other of the first and second threshold voltages; and wherein the pulse generating block applies first and second turn-on voltages having the same level such that the floating diffusion node and the channel of the transfer transistor are separated by the depletion region when the transfer transistor is turned on during the period in which photon-induced charges accumulated in the photodiode are transferred to the diffusion node.
14. The image sensor of claim 13 , wherein the threshold voltage of the transfer transistor is made higher than the threshold voltage of the reset transistor.
Cooperative Patent Classification codes for this invention. Click any code to explore related patents in that topic.
December 23, 2009
May 3, 2011
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